Separator for electrochemical cell and process for assembling it into the cell

ABSTRACT

An electrochemical cell, such as an alkaline cell, employing at least two separator strips in which the strips have notches appropriately placed so that when the separator is formed into a closed end cylindrical configuration it will have substantially no folds protruding inside and/or outside its wall at the area adjacent its bottom surface.

This is a divisional of copending application Ser. No. 07/594,525, filedon Oct. 9, 1990, now U.S. Pat. No. 5,154,731.

FIELD OF THE INVENTION

The invention relates to electrochemical cells, such as alkaline cells,which employ a tubular separator closed at one end which is composed ofat least two separator strips each having a bottom segment and extendingsidewalls with the bottom segment of one strip superimposed on thebottom segment of the other strip and wherein notches are formed in theedges of the strip adjacent the bottom segment to substantiallyeliminate the usual projected folds in the edges of the strip that areformed when the strips are assembled into a substantially circularcontainer having an open end. The invention also relates to a processfor assembling the separator in the cell.

BACKGROUND OF THE INVENTION

The subject invention is primarily suited for use with alkaline typecells. Standard alkaline cells are conventionally manufactured with acathode which generally comprises predominantly an oxide depolarizer,such as manganese dioxide usually admixed with a binder and conductivematerial such as graphite and the like. The cathode is generally pressedfirmly against the inside wall of a container with the inner portion ofthe cathode left hollow so that the cathode assumes a closed endcylindrical structure with a centrally defined cavity. Separatormaterial, usually formed into thin sheets of paper-like material, isinserted into the cathode's central cavity and conforms to the cavity ofthe cathode. An anode which usually comprises a consumable anodicmaterial such as powder zinc admixed with a gelling agent such aspolyacrylic acid or carboxymethyl cellulose and a suitable alkalineelectrolyte such as an aqueous potassium hydroxide solution, is thenextruded into the cavity of the separator. Thus the sheets of separatormaterial electrically isolates the anode from the cathode whilesimultaneously permitting ions to flow between the electrodes.Generally, the separator strips can be forced downward and through thecathode's central cavity so that the separator's center region isparallel with and close to the bottom of the container. Occasionally,the insertion of the separator, using a rod driving means, forces thecenter region of the separator into the cathode material at the bottomof the container. On occasions, small pieces of the cathode are driveninto and partially through the separator. This can result in shortcircuit of the anode and cathode of the cell thus rendering the celluseless. In addition, sometimes during discharge of the cell, zincdendrites are formed that can extend through the separator at the bottomregion of the container and short circuit the cell. These problems aredifficult to detect because the cell would have to be disassembled inorder to inspect the bottom central portion of the separator forevidence of cathode mix penetration or dendrite penetration. However,this problem has been substantially eliminated by placing anelectrically insulating barrier, such as a plastic film, on the bottomsurface of the separator. The use of an electrically insulating barrierat the bottom surface of the separator is disclosed in copending patentapplication Ser. No. 560,651 by Kelemen et al filed on Jul. 30, 1990,now U.S. Pat. No. 5,075,958.

British Patent 2,181,584 describes a method of producing a separatorbasket for standard alkaline batteries This process uses a mandrel toform and support a tube of separator material on which the separatormaterial is folded over one end of the tube and then a small amount ofsealant is dispensed on the outside surface of the folded over bottom ofthe separator to form a "basket". The sealant seals the end of theseparator basket and in some applications may secure the separator toother cell components such as the cathode. A final step may includeinsertion of a can containing a molded cathode over themandrel-supported separator.

However, there is a problem associated with using strips of separatormaterial to form a separator basket to be used in cylindrical typealkaline cells. Specifically, one problem in this type of basketseparator is that undesirable folds are formed in the separator stripsnear the bottom segment of the basket. These projected folds orprotrusions are inherent to a process of forming a retaining basket orcavity by inserting two rectangularly shaped strips of material into acircular opening so that the strip segments are aligned andsymmetrically positioned within a tubular shaped electrode (cathode).The undesirable folds occur where the inner most surface of theseparator contacts the bottom area of the electrode. These protrusionsof folded material, commonly referred to as "ears", may extend to theinside and/or outside of the separator basket. These protrusions are anunavoidable part of a process which forces strips of uniform width intoa circular type opening whose inside diameter is less than the width ofthe strips.

At least two problems can be caused by the presence of theseprotrusions. In alkaline cells, if the anode mix flows into the gaps orfolds of the protrusions, the zinc in the anode may work its way betweenthe strips and eventually cause an electrical short within the cell. Thesecond problem pertains to folds in the separator that protrude into theretaining cavity thereby preventing the anode from falling to the bottomof the retaining cavity. This problem can force the trailing end of theanode to project above the top of the cathode and thereby create asituation in which the anode can be easily smeared onto the cathodeduring the remainder of the cell assembly process. In addition, theremoval of the excess protrusion material will permit more activematerial to be placed into the cell and thereby provide greater outputcapacity.

It is an object of the present invention to provide a separator basketfor electrochemical cells that is made from separator strips of materialand wherein said separator basket has no effective protrusions or foldsthereby providing a more uniform cavity.

It is another object of the present invention to provide a separatorbasket made from rectangular strips of material which have notchesappropriately formed in the edges of the strips so that the strips uponbeing assembled to form a separator basket will effectively be free ofany undesirable folds protruding from the wall of the basket.

It is another object of the present invention to provide a separatorbasket that is easy to make and cost effective to produce.

It is another object of the present invention to provide a process forassembling the separator basket into a cell.

These and other objects and advantages of the invention will be apparentfrom the following description and drawings.

SUMMARY OF THE INVENTION

The invention relates to an electrochemical cell assembled in a housingcomprising a container having a closed end and an open end closed by acover; a first active electrode material positioned within andcontacting the inner wall of the container and defining a centrallydisposed cavity; a separator comprising at least two separator stripswith each strip having a bottom segment with extending walls and eachwall being defined by two edges; the bottom segment of one stripsuperimposed on the bottom segment of the other strip with the extendingwalls of each strip arranged to face the edges of the walls of the otherstrip so that the walls define a cavity in said separator and whereineach edge of at least one strip has a notch formed in the area of theedge adjacent the bottom segment of the strip to substantially eliminatethe usual projected folds in the edge of the strip formed at said area;said separator positioned within and contacting said first activeelectrode material; a second active electrode material positioned withinsaid cavity of the separator; a first terminal on the housingelectrically connected to said first active electrode material; and asecond terminal on the housing electrically insulated from the firstterminal and electrically connected to said second active electrodematerial. The area of the bottom segment should preferably beapproximately the area of the cross-section of the cavity of the firstelectrode.

A preferred embodiment of the invention is an electrochemical cellassembled in a housing comprising a cylindrical container having aclosed end and an open end closed by a cover; a first active electrodematerial positioned within and contacting the inner wall of thecontainer and defining a centrally disposed cavity; a separatorcomprising at least two elongated separator strips defined by two spacedapart elongated edges with each edge having two spaced apart notches inwhich the area formed in each strip by the length of the edge betweenthe spaced apart notches and the width between the spaced apart edgesdefine a bottom segment and in which the length of the edge extendingfrom each notch and the width between the spaced apart edges defines awall; the bottom segment of one strip abutting on the bottom segment ofthe other strip and the walls of each strip extended substantiallyperpendicular to the bottom segment with the edges of each wall of astrip superimposed on the edge of a wall of the other strip therebyforming a substantially circular cavity in the separator, the notchesprovide space so that the usual projected folds in the edges of thestrips formed at said area of the separator adjacent the bottom segmentwhen the strips are folded into a substantially circular cavity will besubstantially eliminated; said separator positioned within andcontacting said first active electrode material; a second activeelectrode material positioned within said cavity of the separator; afirst terminal on the housing electrically connected to said firstactive electrode material; and a second terminal on the housingelectrically insulated from the first terminal and electricallyconnected to said second active electrode material.

The invention also relates to a process for assembling a cell using theseparator of this invention which comprises the steps:

(a) positioning the first active electrode material, such as a cathode,inside a container closed at the bottom and open at the top so that thefirst active electrode material defines a centrally disposed cavity,said container being adapted as the terminal for said first activeelectrode material;

(b) preparing a first strip and a second strip of separator materialwith two spaced apart notches in each of the edges of the strips; saidnotches in each edge being spaced apart sufficiently so that thedistance between the notches is not substantially greater than themaximum diameter of the cavity in the first active electrode material;

(c) forcing said first strip of separator material and said second stripof separator material into the cavity of the first active electrodematerial and forcing the strips to assume the contour of the cavity ofthe first electrode material; said first strip being positioned withrespect to said second strip to form within said first electrode aseparator basket which has effectively no projected folds at the areaadjacent to the bottom of the separator basket;

(d) adding a second active electrode material into the cavity of theseparator material; and

(e) sealing the open end of the container with a cover and wherein atleast a portion of the cover is electrically insulated from thecontainer and electrically contacting said second active electrodematerial thereby said portion being adapted as the terminal for thesecond active electrode material.

Preferably the first strip should be positioned about 90° with respectto the second strip and the notches in the first strip superimposed overthe notches in the second strip as to form the separator basket.

The subject invention is designed to prevent the problems associatedwith the formation of undesirable folds in the separator basket. Thebasic concept is to modify the separator strips by cutting out smallportions of the strips prior to inserting the strips into the cathode.Modification of the strips includes cutting out small pieces of theseparators or imparting slits to the strips at those locations where thefolds of the separator create the ears. Preferably, each separator stripshould be notched at least once on each of its longest sides. Thesenotches can be made in a variety of shapes and sizes such as a slit, asemi-circular notch, a triangular notch, or any polygonal shaped notch.The primary criteria to be met are: (1) substantially eliminate any earswithin the retaining cavity after the separator basket has been formed;and (2) there are no direct openings in the separator basket throughwhich the anode can migrate.

If desired a cup shaped piece of material, such as paper, can beinserted into the bottom of the separator basket after the separatorshave been inserted into the cathode. This cup will serve as a backupmeans to insure than anode mix does not work its way through any gapsthat might inadvertently be formed between the edges of the separatorsheets.

If also desired, an electrically insulating barrier layer could bepositioned on the surface of at least one of the bottom surfaces of theseparator basket to prevent internal shorting between the electrodes.Suitable electrically insulating barrier layer materials would includepolypropylene, polyethylene, polyvinyl chloride, asphalt, wax,polyvinylidene chloride, cellophane and nylon. The electricallyinsulating barrier layer could also comprise a film or strip having anadhesive on one of its surfaces so that it could be secured to aselected area (bottom segment) of a separator strip. If the electricallyinsulating barrier material is a two sided type adhesive tape, then itcould be disposed between the bottom segments of the separator stripsthereby keeping them secured during the assembly process. If theelectrically insulating barrier material is plastic, then it could beattached to the bottom surface of a separator strip using heat tolaminate the plastic to the separator or the plastic could be extrudeddirectly onto the separator. In all embodiments, the electricallyinsulating barrier material could provide effective protection againstpiercing of the bottom segment of the separator by small bits of activeelectrode material to prevent the development of an internal electricalshort circuit.

Preferably, the first active electrode material would be the cathodematerial comprising a depolarizer such as manganese dioxide usuallymixed with a binder and conductive material such as graphite, acetyleneblack or mixtures thereof. Preferably, the second active electrodematerial would be an anode comprising zinc admixed with a gelling agentand a suitable alkaline electrolyte such as aqueous potassium hydroxidesolution. Anode materials and their preparation are described in U.S.Pat. Nos. 2,930,064, 2,935,547 and 2,993,947. The disclosure of thesereferences is incorporated herein as if they were presented in theirentirety.

The separator of this invention may be composed of any separatormaterial such as woven or non-woven paper of cellulose fibers laminatedto a similar mat of vinyl fibers, kraft paper, alpha cellulose paper,methyl cellulose film, polyvinyl alcohol, copolymers of polyvinylacetate and polyvinyl chloride, rayon, nylon, and mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of two separator strips with spaced apartnotches showing the first strip superimposed over and folded upward on asecond strip which is positioned 90° to the first strip so that only thecentral areas overlap.

FIG. 2 is a perspective view of the two separator strips after all thenon-overlapping segments (walls) of the strips are folded normal to theplane of the overlapping area of the strips and curved inward to form aclosed end cylindrical configuration.

FIG. 3 is a cross-sectional view of FIG. 2 taken through line 3--3.

FIG. 4 is a sectional view of the separator of FIG. 2 positioned withinthe cavity of a cylindrically constructed cathode.

FIG. 5 is a top view of a sheet of separator material having a series ofspaced apart circular notches formed at the middle area of the sheet.

FIG. 6 is a top view of a sheet of separator material having a series ofspaced apart square notches formed at the middle area of the sheet.

FIG. 7 is a top view of a sheet of separator material having a series ofspaced apart polygonal notches formed at the middle area of the sheet.

FIG. 1 shows a first strip of separator material 2 having walls 10 and12 positioned under and rotated 90° on a second strip of separatormaterial 4 having walls 14 and 16 so that overlapping occurs only inarea 6 Strip 2 has two spaced apart semi-circular notches 5 formed ineach edge 24-26 and edge 28-30 and strip 4 has two spaced apart notches8 formed in each edge 17-18 and edge 20-22. The notches of strip 2 areinitially placed over the notches of strip 4. FIG. 1 shows strip 2partially folded upward so that edges 24 and 28 approach edges 26 and30, respectively. As shown in FIG. 2, when projecting the separatorstrips into a circular cavity of a cathode lined container, walls 10 and12 of strip 2 would be bent normal to area 6 forming a circularconfiguration with walls 14 and 16 of strip 4. The notches 5 and 8formed in the sheets 2 and 4 respectively, will provide space for thefolded strip material and will effectively eliminate the undesirablefolds or protrusions normally formed at the area adjacent the bottomsurface 6 at the mating edges of the walls. The forming of notches 5 and8 in strips 2 and 4, respectively, will provide for a more uniformcylindrically shaped separator basket and eliminate any protrusion ofthe strip materials in the cavity formed in the basket which couldprevent the anode from falling to the bottom of the basket. By foldingwalls 10 and 12 of strip 2 and walls 14 and 16 of strip 4, a cylindricalconfiguration is produced as shown in FIGS. 2 and 3. This cylindricalconfiguration would conform more uniformly to the interior of thecathode pressed firmly against the inside wall of a cell's container Asevident from FIGS. 2 and 3, wall 14 faces edges 24 of wall 10 and edge26 of wall 12; wall 16 faces edges 28 of wall 10 and edge 30 of wall 12;wall 12 faces edge 20 of wall 16 and edge 22 of wall 14; and wall 10faces edge 18 of wall 16 and edge 17 of wall 14 Thus the separatorstrips 2 and 4 are folded during insertion into a cylindrical cavity ofthe cathode and conform to the shape of the cylindrical cavity. As shownin FIG. 4, the overlapping or bottom area 6 is positioned at the bottomof the cylindrical cavity of the cathode 32 in contact with the bottominner surface 34 of cathode 32.

Although the edges of the walls (24-26; 28-30; 17-18; and 20-22) of theseparator strips are shown abutting to provide a circular cylindricalconfiguration, in reality the edges usually overlap to form an irregularcylindrical configuration that is substantially free of folds protrudingat the area adjacent the bottom of the basket. In some applications, theedges may be separated and still provide an overall irregularconfiguration that can be used in some cell applications. The onlyrequirement is that the separator strips provide a cylindrical typebasket having an overall upstanding wall that is completely closed sothat the cathode is not in direct contact with the anode in any area ofthe separator.

FIG. 5 shows a sheet of separator material 40 having a series of spacedapart semi-circular notches 42 formed at its midsection. Sheet 40 couldbe cut into strips 44, 46 etc. as shown by the broken lines to provide astrip 44 that could be used as one of the strips shown in FIG. 1. Ifdesired two such strips 44, 46 could be used as shown in FIG. 1. FIG. 6shows a similar sheet of separator material 50 except that the notchesare spaced apart triangular shaped notches 52 formed from diamond shapedcutouts 54. FIG. 7 shows a similar sheet of separator material 60 exceptthat the notches are spaced-apart polygonal shaped notches 62. Asdiscussed above any shape notch would be suitable as long as it providesa means for substantially eliminating the normal folds protruding fromthe area adjacent the bottom of the separator basket.

A standard alkaline cell can be produced by preparing a quantity ofpowdered cathode material and disposing it into the open end of acylindrical container. A molding ram is then pressed into the powderedmixture that is contained within the container and since the ram'soutside diameter is substantially smaller than the inside diameter ofthe can, an elongated "ring" of cathode mix is molded tightly againstthe container's interior circumference. After the ram is withdrawn, atubular shaped cavity is formed into the central portion of the cathode.Two strips of a separator material with suitable disposed notches areinserted into the cathode's centrally located cavity in order to form aseparator basket. An electrolyte and a gel-like anode are dispensed intothe separator basket and then a seal assembly is inserted into the openend of the container. This assembly includes an elongated currentcollector that projects into the anode and also includes a plasticdisc-shaped seal that fits tightly within the container's opening and isseated slightly below the top of the container. The top of the containeris redrawn until the seal is radially compressed and then the lip of thecontainer is crimped inwardly.

A preferred separator insertion process for standard alkaline cellswould utilize two strip-shaped pieces of separator material withappropriate notches The first step in the separator insertion processinvolves cutting a first strip of separator to an appropriate length andwidth with suitable notches. The length should be equal to at leasttwice the cathode's height plus the inside diameter of the cathode. Thewidth of the strip should be slightly greater than one-half thecathode's inside circumference. Next, the first strip is positioned overthe open end of a container that contains a molded cathode. The strip'sbroad surfaces must be perpendicular to the cathode s longest dimensionand the center point of the separator must align with the cathode'slongitudinal axis. A rod-shaped separator insertion ram is positionedabove the open end of the container. The rod's outside diameter shouldbe slightly smaller than the inside diameter of the cathode's cavity andthe circumference of the ram should be concentric with the circumferenceof the cathode's cavity. As the ram descends it carries the middleportion of the separator downward into the cathode's central cavityuntil the separator touches the inside bottom of the cathode. The twowalls of the separator strip that extend beyond the separator's centralregion extend upward from the bottom of the cathode and line thecathode's sidewalls. The surface of the first strip that contacts thecathode is known as the outside surface of the first strip while theopposite side of the separator is known as the inside surface of thefirst strip A second strip of separator is cut to the correct length andwidth The central portion of the second strip is positioned over theopen end of the cathode that already contains the first separator stripWhen the second strip is positioned above the container. It is rotatedso that after the second strip is inserted into the container, the seamsof the second strip are turned ninety degrees relative to the seams inthe inserted first separator strip. This rotational offset seamarrangement inhibits particles of zinc in the anode mix that couldotherwise work their way through the seams of both separators andthereby create an internal short circuit. After the second strip hasbeen properly located, another rod-shaped insertion ram is positionedabove the cathode and concentrically aligned with the inside diameter ofthe cathode's cavity. The second separator insertion ram descends andinserts the second separator strip inside the previously inserted firstseparator strip. The walls of the second separator extend upward fromthe bottom of the cathode and line the inside surface of the firstseparator. The surface of the second separator that contacts the innersurface of the first separator is known as the outside surface of thesecond separator. The other surface of the second separator faces thelongitudinal axis of the cathode's cavity and is known as the insidesurface of the second separator.

An efficient and reliable separator insertion process is generallycritical to the production of standard alkaline batteries on acontinuous basis. The separator should be consistently inserted to thebottom of the cathode's cavity at the high speeds required by massproduction processes and this must be done without abusing theseparator. The proposed invention is specifically designed to solve theproblem of folds in the formed separator basket from protruding insideand/or outside the walls of the cavity formed by the separator. Thiswill allow the anode mix to be easily inserted into the separator basketand descend down to the bottom surface and prevent any mix from flowinginto the folds since the folds are effectively eliminated.

While the invention has been described in conjunction with specificembodiments, it is obvious that certain modifications may be made to theinvention without deviating from the scope of the invention.

What is claimed is:
 1. An electrochemical cell assembled in a housingcomprising a container having a closed end and an open end closed by acover; a first active electrode material positioned within andcontacting the inner wall of the container and defining a centrallydisposed cavity; a separator comprising at least two separator stripswith each strip having a bottom segment with extending walls and eachwall being defined by two edges; the bottom segment of one stripsuperimposed on the bottom segment of the other strip with the extendingwalls of each strip arranged to face the edges of the walls of the otherstrip so that the walls define a cavity in said separator and whereineach edge of at least one strip has a notch formed in it at the area ofthe edge adjacent the bottom segment of the strip to substantiallyeliminate the usual projected folds in the edge of the strip formed atsaid area; said separator positioned within and contacting said firstactive electrode material; a second active electrode material positionedwithin said cavity of the separator; a first terminal on the housingelectrically connected to said first active electrode material; and asecond terminal on the housing electrically insulated from the firstterminal and electrically connected to said second active electrodematerial.
 2. The electrochemical cell of claim 1 wherein the separatorcomprises at least two elongated separator strips defined by two spacedapart elongated edges with each edge having two spaced apart notches inwhich the area formed in each strip by the length of the edge betweenthe spaced apart notches and the width between the spaced apart edgesdefines a bottom segment and in which the length of the edge extendingfrom each notch and the width between the spaced apart edges defining awall; and wherein the notches at the area adjacent the bottom segmentsubstantially eliminating the usual projected folds in the edges of thestrip formed at said area of the separator.
 3. The electrochemical cellof claim 2 wherein the notches are semi-circular shaped notches.
 4. Theelectrochemical cell of claim 2 wherein the notches are triangularshaped notches.
 5. The electrochemical cell of claim 2 wherein thenotches are polygonal shaped notches.
 6. The electrochemical cell ofclaim 2 wherein a cup is disposed adjacent to the bottom of at least oneseparator.
 7. The electrochemical cell of claim 2 wherein anelectrically insulating barrier layer is positioned on a surface of atleast one of the bottom segments of the separator to prevent internalshorting between the electrodes.
 8. The electrochemical cell of claim 2wherein the first active electrode material is the cathode and thesecond active electrode material is the anode.
 9. The electrochemicalcell of claim 8 wherein the cathode comprises manganese dioxide and theanode comprises zinc.
 10. The electrochemical cell of claim 9 wherein anelectrically insulating barrier layer is positioned on a surface of atleast one of the bottom segments of the separator to prevent internalshorting between the electrodes.
 11. The electrochemical cell of claim10 wherein the electrically insulating barrier layer is a materialselected from the group comprising polypropylene, polyethylene,polyvinyl chloride, asphalt, wax, polyvinylidene chloride, cellophaneand nylon.